Very little is known regarding the factors that affect the development of pain and disability in those with radiographically established osteoarthritis (OA) of the knee. The primary aims of this study are to evaluate the contribution of weakness in the legs (knee flexors and extensors): 1) to the development of symptomatic OA in those with asymptomatic radiographic evidence of OA; and 2) to the loss of function and the progression to disability in those with symptomatic OA. It has been recognized for some time that x-ray evidence of OA of the knee without symptoms is much more prevalent than symptomatic OA, suggesting that factors exist that determine whether or not joint changes become symptomatic. A variety of factors, including long-standing obesity, a history of trauma, and stiffer subchondral bone, have been suggested as risk factors for OA; with the exception of weight loss (which is difficult to maintain for long periods) intervention on these factors is unlikely. In contrast, muscle strength is increasingly being recognized as an alterable risk factor for various disorders associated with aging, including osteoporosis and falls. The public health implications of these findings and the possible importance of strength in OA arise primarily from three areas: 1) the frequency of OA; 2) the established decline in strength, particularly lower extremity strength, with aging; and 3) the potential for older people to increase strength using safe exercise programs over relatively short periods of time. We propose both cross-sectional and longitudinal studies of OA and muscle strength. Healthy adults over age 65 will be recruited from free-living elderly and will undergo baseline assessments to determine: 1) the presence of radiographic evidence of knee OA); 2) the presence of symptoms of OA; 3) knee flexor and extensor strength. At the end of three years, these assessments will be repeated to determine: 1) the development of symptoms of OA; 2) changes in strength; 3) changes in radiographic characteristics. Between these two assessments, each subject will be contacted at six-month intervals to determine whether symptoms of knee OA have developed.